1. Field of the Invention
The present invention relates to TiO.sub.2 pigments having TiO.sub.2 particles with substantially discrete inorganic particles dispersed on the surfaces of the TiO.sub.2 particles. The TiO.sub.2 pigments of this invention exhibit improved light scattering efficiency at a moderate to a high pigment volume concentration (PVC) when the pigment is incorporated in coatings applications. The invention also includes processes for making such TiO.sub.2 pigments.
2. Description of the Related Art
Inorganic pigments, and especially TiO.sub.2 pigments, are often incorporated in various polymers as whiteners, tinting agents or opacifiers. TiO.sub.2 is particularly useful for these purposes because it scatters light very efficiently. However a problem encountered is that as the PVC increases in the polymer, the TiO.sub.2 particles come close to or in contact with other TiO.sub.2 particles which reduces the scattering efficiency of the TiO.sub.2, i.e., there is an optical crowding effect. Typically, scattering efficiency or scattering coefficient is measured by hiding power and tinting strength.
While prior art techniques attempt to minimize the foregoing problem, they are deficient in one or more areas. For example, extenders have been added to paint formulations to space the TiO.sub.2 particles to preserve scattering efficiency, however, these extenders are difficult to distribute within the paint matrix to maximize the TiO.sub.2 hiding power. Extenders are typically larger particles and/or in the form of aggregates and thus tend to increase the effective TiO.sub.2 volume concentration and decrease scattering efficiency as they are added to replace paint resin content as described in F. Stieg, "The Effect of Extenders on the Hiding Power of Titanium Pigments", Official Digest, 1959, pp. 52-64.
Further, hydrous silica with high surface area can be precipitated onto the surfaces of the TiO.sub.2 particles, providing some improvement in scattering efficiency at increasing PVC. However, the TiO.sub.2 pigments with hydrous silica coatings with high surface area have a high oil absorption which degrades the paint film integrity, decreases a critical pigment volume concentration (CPVC) as defined hereinbelow, and restricts a paint maker of flexibility in formulating paints. In addition, silicas such as fumed silica are composed of many primary particles which are highly aggregated and cannot be distributed uniformly on the TiO.sub.2 surface. Similarly, the use of a polymer coating is expensive and difficult to accomplish and has resulted in severe agglomeration of the TiO.sub.2 particles.
Although it is also known to coat metal oxide particles with inorganic a particles by mixing a slurry of the metal oxide particles with a slurry or sol of inorganic particles having a certain particle size, these pigment products are also unsatisfactory for use in coatings and other applications, where the pigment needs to have improved light scattering efficiency.
For example, Morris, U.S. Pat. No. 5,269,840 discloses making colorant clusters by mixing a binder sol with a colorant in particle form to form a pigment sol, and then removing the liquid to form the final colorant clusters. TiO.sub.2 particles are disclosed as being suitable colorants and silica can be used as the sol particles. However, the coating of sol particles (e.g., silica) is described as comprising at least 40% by weight of the coated colorant particle (e.g., TiO.sub.2). Thus, these colorant particles would not be expected to provide good light scattering efficiency in view of such a high weight percentage of silica coating.
Published Japanese Patent Application Kokai Hei 3-275768 discloses a process for surface-treating TiO.sub.2 particles, wherein a slurry of TiO.sub.2 particles and a sol of alumina particles are mixed together to provide a coating of Al.sub.2 O.sub.3 at 1 to 20% by weight based on the weight of the TiO.sub.2 particles. The primary particle size of the alumina particles in the sol is not disclosed. However, the objective of this process is to provide an alumina film coating on the surface of the TiO.sub.2 particles. Thus, it is believed that the primary particle size of the alumina particles would have to be very small in order to produce an effective film coating. These small alumina particles would agglomerate when mixed with the slurry of TiO.sub.2 particles to form a film coating on the surface of the TiO.sub.2 particles. These TiO.sub.2 particles would not be expected to provide improved light scattering efficiency in view of this surface film of alumina.
Patterson, U.S. Pat. No. 2,187,050 discloses a process for coating TiO.sub.2 pigment by first forming a precipitate of hydrated aluminum oxide, purify it, then preparing a slurry or suspension of the alumina and blending this slurry or suspension with a slurry or suspension of TiO.sub.2 pigment. The mixed slurry is then filtered, and the TiO.sub.2 pigment is dried. The particle size of the alumina particles in the alumina slurry or suspension is not disclosed. Weber, U.S. Pat. No. 4,376,655 also discloses a process for coating TiO.sub.2 pigment by blending an amorphous aluminum hydroxide gel with TiO.sub.2 particles and water to provide an aqueous slurry of TiO.sub.2 pigment. This process improves the optical efficiency of the TiO.sub.2 pigment slurry for application to paper products or other coating applications. The particle size of the alumina particles in the aluminum hydroxide gel is not disclosed. However, gels generally have interconnected particles and thus discrete particles do not form on the surfaces of the TiO.sub.2 particles.
Published Japanese Patent Application Kokai No. 2-51419 discloses a process, where CaCl2 is dissolved in a slurry of TiO2 particles, NH4OH is introduced, and then CO2 gas is introduced into the slurry. Published Japanese Patent Application Kokai No. 2-51419 also discloses a TiO2 pigment product having a film-like coating of CaCO3 at 50 wt. % based on the weight of the coated pigment. A film-like coating of CaCO3 is described as not being obtained by the processing methods used in Comparative Example 1.
Published UK Patent Application GB 2 267 503 A describes a process for making a composite particulate pigmentary material, wherein a dispersion of a particulate pigmentary material (e.g., TiO.sub.2 particles) having a positive or negative surface charge is mixed with a dispersion of a second particulate material (e.g., polymer microspheres, silica, silicates, aluminates, sulphates, carbonates, or clays) having a surface charge with an opposite sign. The average size of the non-pigmentary particles is described as being preferably between 0.02 and 0.3 microns. The ratio of TiO.sub.2 to non-pigmentary particles is described as being preferably in the range from 0.3:1 to 3:1 by volume.
There is a need for a TiO.sub.2 pigment product that has improved scattering efficiency at moderate to high PVC. Concomitantly, were is a need for a TiO.sub.2 pigment having a low oil absorption so that the paint film integrity can be maintained. The present invention meets these needs.